1. Field of the Invention
The present invention relates generally to integrated circuits (ICs), and more particularly, to a method and apparatus of providing a programmable slew rate control output driver for an IC.
2. Description of the Related Art
Typical integrated circuits (ICs) include a set of output drivers for driving signals provided by internal circuitry of the IC through output pads onto output transmission lines, which may be individual lines of an output bus. A typical output driver includes one or more transistors connected to a power supply and ground. Gates of the transistors are connected to internal lines providing the signals to be output.
Because output signals are often used as inputs to subsequent logic devices, the output signals must have sufficient voltage and current to drive the subsequent devices. A common practice is to buffer output signals by using transistors to ensure adequate output signal strength. However, providing each output signal with its own output buffer and driver, supplied by a power supply can create noise problems when many outputs switch state at once. For example, when several output buffers switch from one binary state to another, the output buffers and drivers tend to draw significant current (even CMOS buffer and drivers tend to draw current when the buffers switch states), which in turn tends to cause, through the inductance of the package of the IC, significant noise from the switching of signals on the output conductors of the package. Moreover, the reference voltages, such as V.sub.cc and V.sub.ss, supplied on the IC tend to deviate from their normal voltage levels when many outputs switch state at once. These problems increase when the switching rate of operation for the buffers and drivers, or the number of buffers and drivers that are switching, increases; that is, when the frequency of the switching increases, more noise results and more deviations of V.sub.cc and V.sub.ss (from normal voltage levels) also results.
One current approach to these problems has involved the use of terminating or dumping resistors at the output of an IC. These dumping resistors are configured to reduce signal ringing problems. However, the addition of components to the IC reduces the signal quality and reliability. In addition, such terminating or dumping resistors are expensive to implement.
Accordingly, there is a need in the technology for an apparatus and method of providing a programmable slew rate control output driver for an IC to permit the designer or chip user to reduce noise, power consumption, or increase the speed of the IC, without sacrificing performance.